Advertisement

Advertisement

New Scientist Live

Two noses are better than one

By Peter Aldhous

TO BE equally at home on land and in water amphibians double up some of their faculties, according to a team of German physiologists. One species of toad, at least, seems to have two senses of smell – one for use in the air, the other adapted for life underwater.

Heinz Breer and his colleagues at Hohenheim University in Stuttgart studied genes that code for the proteins carried on the surface of receptor cells, which bind to smelly molecules. In rats, where receptor genes of this type were first identified, there are around 1000 varieties. Catfish have a range of similar but subtly different genes, but only about 100 of them.

When the Stuttgart researchers started looking at the African clawed toad, Xenopus laevis, which lies between fish and mammals on the vertebrate evolutionary tree, they were expecting to find an intermediate number of genes, with sequences looking like hybrids of those from fish and rats. Instead, they saw that the toads had two distinct families of receptor genes. One group resembled those from catfish, and the other looked like the rat genes (Neuron, vol 15, p 1383).

Breer and his colleagues realised that Xenopus could have a dual sense of smell. They speculated that the mammal-type genes coded for proteins adapted for detecting airborne odours, while the fish-type receptors were for smelling underwater.

Advertisement

The best evidence came when Breer’s team looked to see which cells carried which type of receptor. The cavity behind a toad’s nostril contains two sacs, called the main and lateral diverticula. These are opened or closed by a flap of tissue that sits between them. In the air, the main diverticulum is open and the lateral is closed. Underwater, the lateral opens and the main is sealed off. Xenopus is largely aquatic, Breer says, but it still regularly takes the air. “What they do is put their nose out of the water.”

The researchers made sequences of RNA – the intermediate step between gene and protein – that bind to the RNA produced by each type of receptor gene. By washing these probes over the toads’ nasal cavities and determining where they bound, they showed that the fish-type receptors were carried in the lateral diverticulum and the mammal-type receptors in the main. Breer believes that a similar arrangement will also be found in other amphibians.